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Corrosion & Prevention 2012 Paper 167 - Page 1
THERMAL METAL SPRAY: SUCCESSES,
FAILURES AND LESSONS LEARNED
Willie L Mandeno
Opus International Consultants Ltd, Wellington, New Zealand
SUMMARY: Thermally sprayed metal (TSM) includes proven long term protective coating systems for steelwork in a marine environment such as thermal sprayed zinc (TSZ) and thermal sprayed aluminium (TSA); however specifiers have been slow to adopt these in Australia. This paper reviews the technology then looks at several projects in New Zealand and overseas, some where a premature failure has occurred, and discusses these and the lessons that should be learned. It concludes with recommendations as to how coating specifications could be improved so that TSM’s potential long life performance can be achieved.
Keywords: Thermal metal spray, Protective coating, TSZ, TSA, Specification
1. INTRODUCTION
The process of spraying molten metal onto steel was first patented in Switzerland by Dr Schoop and introduced to the UK
in 1912, but did not become a commercial reality until the early 1920's [1]. Metal spraying of bridge components (e.g. the
Menai Straits Bridge) was carried out in Britain before World War II. Because of the significant reduction in
maintenance, flame sprayed zinc supplemented by paint became widely specified by British engineers for many major
structures around the world, including the Auckland Harbour Bridge (1958), the Forth Road Bridge (1964), and the
Pierre-Laport suspension bridge across the St Lawrence at Quebec (where from 1978-84 some 165,000 sqm was coated
after failure of the original paint system) [2]. Use of flame sprayed zinc as a primer declined with the introduction of the
self-curing types of inorganic zinc silicates in the 1970's and has since mainly been used to coat steel components that
could not be “galvanized”, ie by dipping into a bath of molten zinc. Typical production deposition or melt rates of zinc
wire when flame spraying were 10-20 kg/hr.
The arrival of arc spray technology in the mid 1960's greatly increased coating adhesion and gave typical application rates
of 10-35 kg/hr using a 2.5 mm maximum sized wire, but the finish of the sprayed metal was rougher and spray
efficiencies of 50% were typical. In 1990 high deposition low energy systems became available which were also much
lighter and portable. These give deposition rates of 20-90 kg/hr with deposition efficiencies of over 70% when arc
spraying 4.8 mm wire. However there was an inherent problem of arc shorting when spraying large diameter wire at low
amperages (250-400 A) which was overcome by a patented control system that can adjust the arc gap while spraying in
less than 0.02 of a second [3]. This and other recent developments in atomisation control giving smoother finishes have
reduced application costs and have made thermal metal spray a very competitive long life coating system.
A description of the main processes involved in thermal spray, and a discussion of the advantages and disadvantages of
TSM as a protective coating for steel when compared to galvanizing or coating with inorganic zinc silicate, have been
given in an earlier paper by the author [4].
2. EXPOSURE TRIALS
In North America the TSM process is known as “metallizing” with early work being carried out by the American Welding
Society who in 1953 exposed panels coated with flame sprayed zinc and aluminium and various sealers. Very favourable
results were reported after 19, 34 and 44 years of exposure at coastal and industrial sites [5,6,7]. This work was followed
Corrosion & Prevention 2012 Paper 167 - Page 2
up by the US Corps of Engineers with successful trials of TSM as a more abrasion resistant coating than vinyl on dam
gates, and which resulted in a comprehensive design manual [8] which is available on the internet.
The US Federal Highway Agency (FHWA) noted [9] that work by the AWS and US Navy showed that “properly applied
metallized coatings (zinc, 85% Zinc/15% Aluminum, and Aluminum) of at least 6 mils thickness provide at least 20 years
of maintenance free corrosion protection in wet, salt-rich environments and are expected to provide 30 years of protection
in most bridge exposure environments”. The FHWA has sponsored several research projects coating steel bridge beams
with TSM, including one of environmentally acceptable materials which found that the TSZ systems were the best
performing over 40 coating systems tested (which included uncoated and topcoated “high -ratio” and other IOZ’s) with no
undercutting at scribe marks after 6.5 years exposure, and had the lowest Life Cycle Cost [10].
Thermal Sprayed Aluminium (TSA) has been widely used in offshore oil and gas industry and by 1997 over 400,000 sq
metres of TSA had been applied to oil platforms in the North Sea [11] to provide corrosion protection to flare stacks, riser
pipes in the splash zone and submerged tethered legs (e.g. Conoco’s Hutton platform built in 1984). Experience
indicated that TSA coatings, when properly applied and with the use of specific sealer systems, will provide a service life
in excess of 30 years with zero maintenance required [12].
3. STANDARDS
The two main standards available for local specifiers are the International Standard ISO 2063 [13] (with its various
national equivalent versions such as BS EN ISO 2063) and the North America joint Standard, NACE 12/AWS
C2.23M/SSPC-CS 23.00 [14]. These are supported by other standards that cover guidance in such matters as
specification [15], operator qualification [16], and adhesion testing [17]. In addition, individual groups such as oil
companies and military organisations have their own in-house specifications [18,19].
Expected durability of TSM systems in different atmospheric environments have been given in recent versions of AS/NZ
2312 which were initially derived from international sources such as BS 5493 [20], ISO 14713 [21] and ANSI/AWS 2.18
[15]. These expected lives to first major maintenance (when breakdown exceeds 2% rust) are given in the Tables below.
The suffix ‘S’ denotes a sealed coating with the TSM thickness given in microns. Note that these tables are likely to be
further amended as part of the current revision of AS/NZS 2312.
Table 1: AS/NZS 2312:1994 [22]
Atmospheric
Classification
Years to first major maintenance
>2 to 5 >5 to 10 >10 to 20 >20 to 25 >25 to 40 >40
Moderate - - ZN100 ZN150 ZN175 ZN300
Marine - ZN100
ZN75S
ZN150
ZN100S
ZN150S
AL150
ZN350
AL275
ZN375S
AL275S
Severe Marine ZN100 ZN150 ZN250 AL275 AL325
Table 2: AS/NZS 2312:2004 [23]
System
designatio
n
Super-
seded
design-
nation
Nominal
coating
thicknes
s
um
Durability – Years to first maintenance
Atmospheric corrosivity category
A Very
low
B
Low
C
Medium
D High E-I Very
high
indust-
rial
E-M
Very
high
marine
F
Inland
Tropical
TSZ100 ZN100 100 25+ 25+ 25+ 15-25 NR 5-15 25+
TZS150 ZN150 150 * * 25+ 25+ NR 10-25 25+
TSZ200A ZN200S 200+seal * * 25+ NR 25+ *
TSA150S AL150S 150+seal * * 25+ 25+ 15-25 15-25 25+
TSA225S AL225S 225+seal * * * 25+ 25+ 25+ 25+
NR = Not recommended * Very high durability but unlikely to be economic
Corrosion & Prevention 2012 Paper 167 - Page 3
4. SEALING AND PAINTING OF TSM
Thermal sprayed metal, and zinc in particular, can have their performance in marine environments enhanced by sealing
the pores and surface with a low viscosity material with good wetting properties such as vinyl, acrylic, or thinned epoxy or
urethane. Silicone aluminium is used for high temperature applications. This is applied soon after spraying the metal
and until absorption of the sealer is complete. Use of aluminium or coloured pigments can give a more uniform
appearance of the coating, while the binder decreases exposure of the metal to the environment. High build paint coatings
are unnecessary and as for galvanizing can reduce the system life by trapping salts and moisture against the metal at
coating defects [24]. The increased life due to sealing is shown in the table above, where sealing zinc is approximately
equivalent to adding a third more metal thickness. Sealing of aluminium has less effect on its durability but is usually
specified to prevent initial rust bleed through that may occur when a film of less than 150 microns is exposed to rain
before pores between the splats have been filled with oxidation products.
5. AUSTRALASIAN EXPERIENCE WITH TSM
5.1 Discussion
Despite the potential for long life protection of assets in marine environments using TSM, its higher initial cost has meant
it has not been widely accepted by specifiers in Australia. This reluctance has been reinforced by problems on contracts
due to lack of experience or expertise, and some premature failures that have resulted for various reasons, including poor
quality control and unsuitable specifications. These are discussed in the remainder of this paper.
5.2 New Zealand Applications
The first major application was on the Auckland Harbour Bridge when in 1956 its coating specification was amended to
the “best protective treatment known at the time”, i.e. flame sprayed with zinc at 50 microns and top coated with 3 coats
of phenolic paint [25]. Another early and successful application was in 1960 to the roof trusses to the main grandstand at
the Ellerslie racecourse in Auckland [4]. There have been many other applications of TSZ to steel items such oil piping,
lighting columns, balcony supports; generally to components that could not be hot-dip galvanized.
Recent applications have ranged from use of sealed TZA on wharf piles in Lyttleton and a highway bridge over
geothermal steam pipelines at Wairakei. Sealed TSZ has been used to maintain in situ RSJ bridge beams over Ship Creek
adjacent to a West Coast surf beach near Haast and also to coat new steel box girders on the Newlands Interchange Bridge
in Wellington. Duplex systems (TSZ with epoxy/polyurethane topcoats) have also been applied to several new bridges
including the replacement Kopu Bridge on State Highway 25 near Thames, the iconic Te Rewa Rewa Footbridge near
New Plymouth, some smaller footbridges such as at Petone Railway Station, and seismic strengthening under the Thordon
Over Bridges in Wellington. Some of these are pictured below and others that had problems are discussed in more detail
in a later section.
Figure 1. Auckland Harbour Bridge (1958) Figure 2. Wairakei Bridge (2010)
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Figure 3. Ship Creek (2003) Figure 4. Newlands Bridge (1999)
Figure 5. Te Rewa Rewa Footbridge (2010) Figure 6. Kopu Bridges (2011)
5.3 Australian Applications
An early and successful use of TSM in Australia was on the second Warragamba pipeline (3m diameter and 22km long)
built between 1964 and 1969. The first had been coated with the same heat-cured zinc silicate used on the Morgan-
Whyalla pipe line. While these have since been repainted several times, this has been due to delamination of the various
overcoating systems and the underlying TSZ is still sound [26]. These pipelines supply 80% of Sydney’s water supply.
Possibly the largest recent TSM project that has been carried out was in 1998 at the Port Stanvac Mobil Refinery
(currently being demolished) near Adelaide. Some 11,000 sqm of new pipe spooling and structural steel was coated with
275 microns of TSA and 1500 sqm of existing steelwork was coated in situ with 350 microns of TSZ. Problems that
occurred on this project due to applicators inexperience with arc spraying were discussed by Naylor [27] at the 1999 ACA
conference.
5.4 Other Australasian Applications
Eleven tonnes of flame sprayed zinc powder was used in 1976 to coat the radial spillway gates on the Late Dam near
Noumea in New Caledonia which are reported to still be in excellent condition, as are LPG spheres beside the harbour
[28]. A TSA duplex system was recently used to coat a gas platform near East Timor but this suffered premature failure
which is discussed later in this paper.
Corrosion & Prevention 2012 Paper 167 - Page 5
Figure 7. Warragamba Pipeline (P2) Figure 8. Port Stanvac refinery
Figure 9. Late Dam gates, New Caledonia Figure 10. Gas and oil storage, Noumea
6. FAILURES WITH TSM
6.1 LPC Coal Berth Piles.
TSA has been used since 2005 to protect structural steel used as beams and jackets to strengthen old timber wharves
owned by the Lyttleton Port Company (LPC) and this has performed well where correctly applied (Figure 11).
In 2008 the author was asked to investigate premature failure of TSA coating on a new coal loading berth for the
Lyttleton Port Company (LPC). As shown in Figure 12, the aluminium coating had been removed in the tidal zone on
most of the piles; some were more severely affected than others. The piles were designed as bare steel protected with
sacrificial anodes with a sealed TSA coating to protect them above the low tide level. Inspection by divers found that the
installed anodes were undersized for the area they were required to protect and some had become disconnected during
driving. The TSA was therefore acting as a sacrificial anode and had to be replaced by a petrolatum wrap and HDPE
jacket, plus bigger capacity anodes were retrofitted to the piles to protect the uncoated surfaces.
Corrosion & Prevention 2012 Paper 167 - Page 6
Figure 11. TSA on pile jacket Figure 12. TSA “failure” on pile
6.2 Ahururi Bypass piles
An expressway near Napier Airport includes a bridge structure over the Ahuriri Lagoon. The steel jackets to its concrete
piles were coated with TSA325 in 2003. A few years later roughly circular rust patches approx100 to 200mm in diameter
appeared in random locations below high tide level as shown in Figure 14. Its failure is currently under investigation
with unsuitable repair methods (patching areas of low build with UHB epoxy) being suspected as being responsible.
Figure 13. Ahuriri Bypass Figure 14. Failure of TSA repair
6.3 TOB Catch frames
Seismic strengthening was carried out on the Thorndon Overbridges (TOB) in 1998. This elevated motorway structure is
located beside the SW corner of Wellington Harbour and where it crosses the Wellington Fault, “catch frames” were
installed to eight pier heads, and steel jackets were added to improve the capacity of all the concrete piers. The seventy
steel jackets were coated with TSZ300 and sealed with an aluminium vinyl. The catch frame steelwork was coated with
TSZ150 and overcoated with 50 microns of epoxy and a 50 micron finish coat of polyurethane.
The seal coating is delaminating from the upper levels of the pier jackets where windborne marine salts are not removed
by rain washing (Figure 16) but no corrosion has been observed. However red rusting has occurred on the catch frame
steelwork which is currently being recoated but with an additional 200 micron epoxy intermediate coat added to the
system over an epoxy zinc patch primer. The corrosion is due to delamination of the duplex TSZ from flange edges of the
fabricated I-beams and also pitting corrosion where peaks of the zinc had inadequate barrier protection from the epoxy
urethane (Figure 15).
Corrosion & Prevention 2012 Paper 167 - Page 7
Figure 15. Duplex TSZ failure on catch frame Figure 16. Seal coat delaminating on TSZ
6.4 Petone Station Footbridge
This structure, located 600m from the Wellington Harbour, was coated with a duplex TSZ system in 2010 but required
patch painting within two years. The specified system was 175 microns of TSZ, sealed with 50 microns of epoxy and
topcoated with 75 microns of polyurethane. Premature failure occurred in areas where the peaks of the coarse surface
profile of the TSZ had not been smoothed as required by the specification and so were close to the top surface of the paint
on surfaces that were sheltered from rain washing.
Figure 17. Petone Footbridge Figure 18. Failure of Duplex TSZ
6.5 Millenium Footbridge
Built as part of a waterfront walkway near Mission Bay, Auckland in 2001, the steelwork that was designed by artist
Virginia King, was coated with TSZ. GANZ has reported [29] that this failed prematurely and in 2009 the flaking TSZ
was removed and replaced with a duplex galvanizing system at a cost of $70,000.
Figure 19. Millenium Footbridge Figure 20. Millenium Footbridge
Corrosion & Prevention 2012 Paper 167 - Page 8
6.6 Gas Platform
The duplex TSA coating on this platform in the Timor Sea began failing within a year of its construction and is currently
undergoing extensive maintenance painting. Similar problems have been found on North Sea platforms where the
performance of duplex TSA has been inferior to the excellent performance of sealed TSA. This was investigated by
SINTEF [30] and failure is attributed to the formation of hydrochloric acid from unstable aluminium chloride trapped
under the organic coating.
Figure 19. Gas platform maintenance painting Figure 20. Hydrogen bubbles from duplex TSA
7. DISCUSSION
The above failures illustrate several lessons that protective coating specifiers and inspectors need to be aware of, in order
for the potential long life of TSM coatings to be achieved.
7.1 Cathodic Protection
TSZ and TZA will provide cathodic protection (CP) to small coating defects and will protect small bare areas. However
when used on structures that are immersed in seawater, their life as a barrier coating will be significantly reduced when
connected to large areas of bare steel (or more cathodic metals) which must have a correctly designed and installed CP
system to protect them, and also to ensure the TSM remains at a passive potential.
7.2 Duplex TSA
While sealed TSA performs better in salt water immersion and in the splash zone than sealed TSZ, the SINTEF study
demonstrated that in marine environments “TSA should not be painted with thick protective coatings” due to the
formation of an acidic electrolyte under the coating. As aluminium is not passive at pH <4 it corrodes actively with
cathodic evolution of hydrogen gas and regeneration of the acidic environment. Formation of unstable aluminium
chloride was observed to not occur when TSA is sealed with a thin single coat which if <80 microns is thought to have
low ionic resistance [31].
7.3 Duplex TSZ
Overcoated TSZ in seawater forms zinc chloride which is relatively stable, soluble and does not acidify the electrolyte.
However when any zinc protective coating is overcoated with an organic coating, its ability to self-protect itself and any
breaks in the coating with a large anodic surface is correspondingly reduced. This applies to TSZ just as it does to
galvanized steel and inorganic zinc silicate coatings. Where these zinc coatings need to be coated for chemical or
abrasion resistance, or to change their colour for safety or aesthetic reasons, it is important that a continuous and low
permeability coating is applied of sufficient thickness to provide an effective barrier to corrodent materials, especially
where these are not removed by rain washing.
SINTEF reported that duplex TSZ had performed well on several road bridges in Norway where four coats of paint were
applied over TZS100. A duplex TSZ system has been used successfully on New Zealand made air-bridges that have been
supplied to several airports around Australasia (Figures 21 & 22). The future performance of the duplex coatings on the
Te Rewa Rewa and Kopu bridges (Figures 5 & 6) will be monitored with some interest.
Corrosion & Prevention 2012 Paper 167 - Page 9
Figure 21. A380 loading in Sydney Figure 22. Duplex TSZ applied in NZ
7.4 Surface profile
TSM applied by arc spray equipment that has been set to achieve a high production rate may also produce a coarse surface
profile. This may be desirable where a non-skid surface is required, but when it is to be part of a duplex system it is
important to smooth the TSM surface by sanding followed by vacuuming prior to sealing. This will remove the ‘rogue
peaks’ that may initiate premature corrosion at areas of low build. AS/NZS 2312 recommends a profile height of less
than 50 microns for TSZ.
The surface profile of the steel substrate being sprayed is also important to ensure adequate adhesion. A very clean
surface with a sharp angular profile of at least 50 microns is required for TSZ, and at least 75 microns for TZA. This may
not be achieved on the flame cut edges of flanges on fabricated plate girders, unless the local surface hardening at gas cut
edges are removed by grinding prior to abrasive blasting. Sections cleaned using shot in a centrifugal blaster such as a
Wheelabrator, will require a final blast with grit to achieve a suitable profile shape. The author has investigated
delamination of TSZ from a pier hand railing where the surface profile was measured at <30 microns.
7.5 Specification
As with any coating system it is important that the owner’s requirements for all stages of surface preparation, application,
inspection and repair are clearly established in advance and then confirmed as being carried out by a suitable QC/QA
system. Reference to ISO 2063 or NACE 12/AWS C2.23M/SSPC-CS 23.00 is recommended.
Also while TSM is capable of providing excellent protection, there are some intricate steel structures where hot-dip
galvanizing may be a more appropriate coating system due to a lower risk of incomplete coverage.
7.6 Quality of application
In addition to the factors already discussed that can lead to premature failure of TSM, it is also necessary to stress that
application of TSM requires a greater level of applicator skill than the spraying of wet protective coatings. The
equipment setup and stand-off distance needs to be optimised to minimise porosity of the TSM, which will then maximise
its cohesive and adhesive strength, and durability. Being a single coat system, it is important that application is carried
out systematically to ensure that there are no areas of low build that can occur if there is insufficient overlap between
spray patterns. Ideally a TSM applicator should be certified as competent to apply the different materials with the various
different processes, in the same way as a welder is certified. Operator certification is the norm in Europe and North
America and the introduction of a similar scheme into Australasia is currently being investigated by the ACA. This
would go a long way to improving the confidence of specifiers that the potential performance of these coating systems will
be realised.
8. ACKNOWLEDGMENTS
The support of Opus International Consultants Ltd to allow presentation of this paper is thankfully acknowledged.
Thanks also go to Marc Herring, Bill Koelman, Daniel Lillas, Fred Salome, Jacques de Reuck, and www.flickr.com for
provision of additional photographs to supplement those taken by the author.
Corrosion & Prevention 2012 Paper 167 - Page 10
9. REFERENCES
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Technology, March 1996.
2 R. Snook, “Corrosion Control by Zinc Thermal Spraying”, Third World Congress on Coatings Systems for
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Aluminium using a High Deposit Rate Machine”, SSPC Conference, San Diego, USA, November 1997.
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years of Inorganic Zinc Coatings’ ed. R Francis, published by Australasian Corrosion Association, 1997.
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Beach”, LaQue Centre for Corrosion Technology Inc, February 1996. (Also referenced on page 72 of ASTM
STP 1399, 2000).
8 US Army Corps of Engineers, Engineer Manual 1110-2-3401, “Thermal Spraying: New Construction and
Maintenance”, Jan 1999.
9 FHWA Bridge Coatings Technical Note: “Metallized Steel Bridge Coatings”, January 1997.
10 R. Kogler, P. Ault & C. Farschon, “Environmentally Acceptable Materials for the Corrosion Protection of
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Conference, New Orleans, April 2004.
13 ISO 2063:2005, “Thermal spraying: Metallic and other organic coatings. Zinc, aluminium and their alloys”
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(Metallizing) of Aluminium, Zinc, and their alloys and composites for the corrosion protection of steel”,
NACE International, 2003.
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20 BS 5493:1977, “Code of practice for protective coating of iron and steel structures against corrosion” (Section
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Corrosion & Prevention 2012 Paper 167 - Page 11
21 ISO 14713:1999 Protection against corrosion of iron and steel in structures - Zinc and aluminium coatings
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protective coatings”, incorporating Amendment No.1:2004
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10. AUTHOR DETAILS
Willie Mandeno is the Technical Principal: Corrosion Engineering and Protective
Coatings at Opus International Consultants Ltd in their Wellington office in New
Zealand. He is a past ACA President, an ACA Corrosion Medallist, a NACE
Protective Coating Specialist, and has been an instructor for over 25 NACE CIP
training courses.